class_docstr: RCNN bbox head
  
  Args:
      head (object): the head module instance, e.g., `ResNetC5`, `TwoFCHead`
      box_coder (object): `BoxCoder` instance
      nms (object): `MultiClassNMS` instance
      num_classes: number of output classes
code: def __init__(
      self,
      head,
      box_coder=BoxCoder(),
      nms=MultiClassNMS(),
      bbox_loss=SmoothL1Loss(),
      num_classes=81,
  ):
      super(BBoxHead, self).__init__()
      self.head = head
      self.num_classes = num_classes
      self.box_coder = box_coder
      self.nms = nms
      self.bbox_loss = bbox_loss
      self.head_feat = None
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from collections import OrderedDict
  from paddle import fluid
  from paddle.fluid.param_attr import ParamAttr
  from paddle.fluid.initializer import Normal, Xavier
  from paddle.fluid.regularizer import L2Decay
  from paddle.fluid.initializer import MSRA
label_desc: Use image node_id 1 to create a new BBoxHead object from inherited base classes: object with example: obj = BBoxHead(head, box_coder, nms, bbox_loss, num_classes)